feat: auto cache inline prop literals to avoid child re-render

flow/component.js

@@ -69,6 +69,7 @@ declare interface Component {
   _staticTrees: ?Array<VNode>; // v-once cached trees
   _hasHookEvent: boolean;
   _provided: ?Object;
+  _inlineComputed: ?{ [key: string]: Watcher }; // inline computed watchers for literal props
 
   // private methods
 
@@ -129,6 +130,8 @@ declare interface Component {
   _k: (eventKeyCode: number, key: string, builtInAlias?: number | Array<number>, eventKeyName?: string) => ?boolean;
   // resolve scoped slots
   _u: (scopedSlots: ScopedSlotsData, res?: Object) => { [key: string]: Function };
+  // create / return value from inline computed
+  _a: (id: number, getter: Function) => any;
 
   // SSR specific
   _ssrNode: Function;

src/compiler/parser/index.js

@@ -29,16 +29,20 @@ const argRE = /:(.*)$/
 const bindRE = /^:|^v-bind:/
 const modifierRE = /\.[^.]+/g
 
+const literalValueRE = /^(\{.*\}|\[.*\])$/
+
 const decodeHTMLCached = cached(he.decode)
 
 // configurable state
 export let warn: any
+let literalPropId
 let delimiters
 let transforms
 let preTransforms
 let postTransforms
 let platformIsPreTag
 let platformMustUseProp
+let platformIsReservedTag
 let platformGetTagNamespace
 
 type Attr = { name: string; value: string };
@@ -66,9 +70,11 @@ export function parse (
   options: CompilerOptions
 ): ASTElement | void {
   warn = options.warn || baseWarn
+  literalPropId = 0
 
   platformIsPreTag = options.isPreTag || no
   platformMustUseProp = options.mustUseProp || no
+  platformIsReservedTag = options.isReservedTag || no
   platformGetTagNamespace = options.getTagNamespace || no
 
   transforms = pluckModuleFunction(options.modules, 'transformNode')
@@ -529,6 +535,15 @@ function processAttrs (el) {
             )
           }
         }
+        // optimize literal values in component props by wrapping them
+        // in an inline watcher to avoid unnecessary re-renders
+        if (
+          !platformIsReservedTag(el.tag) &&
+          el.tag !== 'slot' &&
+          literalValueRE.test(value.trim())
+        ) {
+          value = `_a(${literalPropId++},function(){return ${value}})`
+        }
         if (isProp || (
           !el.component && platformMustUseProp(el.tag, el.attrsMap.type, name)
         )) {

src/core/instance/render-helpers/create-inline-computed.js

@@ -0,0 +1,28 @@
+/* @flow */
+
+import { noop } from 'shared/util'
+import Watcher from 'core/observer/watcher'
+
+/**
+ * This runtime helper creates an inline computed property for component
+ * props that contain object or array literals. The caching ensures the same
+ * object/array is returned unless the value has indeed changed, thus avoiding
+ * the child component to always re-render when comparing props values.
+ *
+ * Installed to the instance as _a, requires special handling in parser that
+ * transforms the following
+ *   <foo :bar="{ a: 1 }"/>
+ * to:
+ *   <foo :bar="_a(0, function(){return { a: 1 }})"
+ */
+export function createInlineComputed (id: string, getter: Function): any {
+  const vm: Component = this
+  const watchers = vm._inlineComputed || (vm._inlineComputed = {})
+  const cached = watchers[id]
+  if (cached) {
+    return cached.value
+  } else {
+    watchers[id] = new Watcher(vm, getter, noop, { sync: true })
+    return watchers[id].value
+  }
+}

src/core/instance/render-helpers/index.js

@@ -10,6 +10,7 @@ import { bindObjectProps } from './bind-object-props'
 import { renderStatic, markOnce } from './render-static'
 import { bindObjectListeners } from './bind-object-listeners'
 import { resolveScopedSlots } from './resolve-slots'
+import { createInlineComputed } from './create-inline-computed'
 
 export function installRenderHelpers (target: any) {
   target._o = markOnce
@@ -27,4 +28,5 @@ export function installRenderHelpers (target: any) {
   target._e = createEmptyVNode
   target._u = resolveScopedSlots
   target._g = bindObjectListeners
+  target._a = createInlineComputed
 }

src/core/instance/state.js

@@ -47,6 +47,7 @@ export function proxy (target: Object, sourceKey: string, key: string) {
 
 export function initState (vm: Component) {
   vm._watchers = []
+  vm._inlineComputed = null
   const opts = vm.$options
   if (opts.props) initProps(vm, opts.props)
   if (opts.methods) initMethods(vm, opts.methods)

test/unit/features/options/props.spec.js

@@ -529,4 +529,38 @@ describe('Options props', () => {
     expect(`Invalid key "reqquired" in validation rules object for prop "value".`).toHaveBeenWarned()
     expect(`Invalid key "deafult" in validation rules object for prop "count".`).toHaveBeenWarned()
   })
+
+  it('should not trigger re-render on non-changed inline literals', done => {
+    const updated = jasmine.createSpy('updated')
+    const vm = new Vue({
+      data: {
+        n: 1,
+        m: 1
+      },
+      template: `
+        <div id="app">
+          {{ n }} {{ m }} <foo :a="{ n: 1 }" :b="{ n: n }"/>
+        </div>
+      `,
+      components: {
+        foo: {
+          props: ['a', 'b'],
+          updated,
+          template: `<div>{{ a.n }} {{ b.n }}</div>`
+        }
+      }
+    }).$mount()
+
+    expect(vm.$el.textContent).toContain('1 1 1 1')
+    vm.n++ // literals that actually contain changed reactive data should trigger update
+    waitForUpdate(() => {
+      expect(vm.$el.textContent).toContain('2 1 1 2')
+      expect(updated.calls.count()).toBe(1)
+    }).then(() => {
+      vm.m++ // changing data that does not affect any literals should not trigger update
+    }).then(() => {
+      expect(vm.$el.textContent).toContain('2 2 1 2')
+      expect(updated.calls.count()).toBe(1)
+    }).then(done)
+  })
 })